1. Field of the Invention
The present invention generally relates to an optical time-division multiplexer for performing time-division multiplexing of optical modulation signals, and more particularly to an optical time-division multiplexer adapted to provide a stable output pattern by regulating the optical phases of two optical modulation signals to be multiplexed.
2. Description of the Related Art
In recent years, with increase in the amount of information transmitted, there have been demands for a high-capacity communication system. Thus, optical communication systems need to be speeded up. To increase the bit-rate at which light (or optical) intensity modulation is performed, the working (or acting) speed of electronic circuit parts must be increased. However, under the present conditions, it is difficult to manufacture an electronic circuit which operates at a high speed, for example, at a bit-rate of more than 20 GHz. To solve such problems, there have been various methods of multiplexing of optical signals (or light signals).
For instance, an optical time-division multiplexer disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 2-167524 has: a light source, such as a semiconductor laser, for outputting light oscillating in a single longitudinal mode; a first optical modulator for modulating an output of the light source, by using a clock signal having a frequency, which corresponds to a half of a desired data rate, and for outputting complementary optical clock pulses whose polarities are opposite to each other; second and third optical modulators for modulating a pair of outputs of the first optical modulator, by using a pair of data signals which are different in phase from each other by 180 degrees and are synchronized with the clock signal, and for outputting only the pulses which coincide with data signals among the optical clock pulses inputted from the first optical modulator; and an optical coupler for synthesizing an optical signal from the optical outputs of the second and third optical modulators. In the case of this optical time-division multiplexer, each of the optical modulators is driven by using the clock signal having a frequency which corresponds to half of the data rate. Thus, the frequency of a drive (or driving) signal for the electronic circuit is lowered. Namely, optical signals are modulated at a frequency that is twice the threshold frequency of the drive signal for the electronic circuit.
In the case of the (conventional) optical time-division multiplexer disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 2-167524, even if two optical signals, which are incident on the optical coupler and are used for synthesizing an output optical signal thereof, are optical complementary pulse signals, the output optical signal changes when the optical phase varies. Under the existing conditions, an optical signal which is obtained when the optical phases of two optical signal to be incident on the optical coupler is different from each other by 180 degrees, is most desirable. Therefore, in the case that such an optical time-division multiplexer is formed on a single circuit board or substrate, the optical time-division multiplexer is designed so that the optical phase difference between two optical signals to be incident on the optical coupler is 180 degrees. However, in the case of actual optical time-division multiplexers, owing to variations in accuracy of the manufacture of the optical time-division multiplexer, it often occurs that the optical phase difference therebetween deviates from 180 degrees. Consequently, the conventional optical time-division multiplexer has a problem in that desirable optical signals are not obtained.
Moreover, in the case that the characteristics or properties of the optical modulator change owing to a variation in temperature, the optical phase difference between two optical signals to be incident on the optical coupler also changes. Therefore, the conventional optical time-division multiplexer has another problem in that even if the optical phase difference between two optical signals to be incident on the optical coupler is 180 degrees in an initial stage and thus a desirable output optical signal is initially outputted therefrom, the output optical signal changes later in response to variation in environmental conditions.